Three guanine nucleotide-requiring reactions occur upon the ribosome during protein chain growth. Two of these, translocation and aminocyl-tRNA binding, are required for chain growth and the third, synthesis of guanosine tetra- and pentaphosphate (the "Magic Spot" nucleotides), is apparently responsible for the regulatory coupling of protein and RNA synthesis. These reactions occupy a central position in the translational process and consequently an understanding of their mechanisms and regulation is essential to an understanding of overall gene expression. We propose to examine and explain a number of operationally and conceptually related problems which distinguish these reactions: 1. Enzymology of guanine nucleotide involvement. 2. Protein chemistry of Elongation Factors G, Tu, Ts and the stringency factor. 3. Nature and relationship of ribosomal sites of interaction. 4. Control of factor synthesis and degradation. 5. Modes of inhibitor functions. 6. Recognition of aminoacyl-tRNA by EF-Tu.